Current technology is allowing the design of larger aircraft than ever before This increase in size results in a corresponding increase in weight and loads which must be carried by its landing gear.
The landing gear on most commercial aircraft includes an "oleo" strut in the form of a nitrogen/oil charged shock absorber. A typical oleo may use a metering rod form of damping with no separation between the nitrogen and oil. Under extreme load, the nitrogen could be compressed, at least theoretically, to zero volume. The high pressure developed by extreme loading can cause the nitrogen to go into chemical solution with the oil. Under certain circumstances, this could result in changes in orifice co-efficients and damping qualities. Additionally, there is a reluctance for the entrapped nitrogen to evacuate the oil as the pressure is reduced. This could result in a temporary variation in static height of the suspension member and the aircraft or other vehicle it supports. When some of the nitrogen in an oleo strut has been compressed into solution and the oleo pressure is released to atmosphere for servicing, nitrogen will effervesce from the oil.
The static load imposed on a landing gear oleo is reacted by the internal pressure distributed over the area of the oleo's piston. Because of the above-described increase in aircraft weight, the area of the piston has had to be correspondingly increased in order to carry the increased load without increasing the static pressure within the oleo cylinder.
Previously, the cylinder of an oleo was charged by filling it with a predetermined volume of oil and then applying pressurized nitrogen from industrial gas bottles on site. In order to service the landing gear while installed in the aircraft, and without jacking the aircraft, nitrogen must be available under sufficient pressure that it will sustain the weight of that aircraft. The maximum specified pressure in standard industrial nitrogen bottles is about 2,400 pounds per square inch (psi) However, for design purposes, and to allow for growth, a maximum pressure of 1,800 psi is used as a design specification to optimize the piston rod diameter. This limitation of static pressure available on site requires an increase of piston diameter as the load requirement increases. Consequently, the outer fitting of the landing gear also increases in diameter, almost proportionally, and the resulting weight and volume of the oleo may be reflected in other fittings of the gear also.